To add scheduled tasks, to a Linux machine, you need to use the crontab command. The following example works for Slackware 8 - it may be different on other versions of Linux.

Create the scheduled task
Locate the crontabs folder, usually located at /var/spool/cron/crontab. In the folder will be a text file with the same name as the currently logged in user. Open that text file in Joe/vi and add the following line to the file:

mm hh dd mm yy /folder/subfolder/yourfile

where mm is the minutes, hh is the hour etc. For example, if you want to run a php file every day at 14:00, you would add the folllowing to the file:

00 14 * * * /www/site/myfile.php

Run the command crontab filename to register the file.
Note: if there is only one line in the file, make sure you have a carriage return at the end of the file or it will fail.
Note: the file must be in Unix format
Note: if you set a scheduled task, for example, to a php file, be sure to add the interpreter directive to the top of the file, e.g.

#!/usr/sbin/php

Delete the scheduled task
Open up the text file and remove the appropriate line from the file. Execute the command crontab filename, for example to register the file.

Alternatively, running the command crontab -d will delete all cron jobs for the currently logged in user.